As it is known by the man skilled in the art, some secure electronic devices, such as some smart cards, have a sensitive mode (such as a test mode) that is used during their production and then disabled to avoid attackers to use it to access to all their resources and thus to control them.
When such an electronic device has a working problem, it may be modified to activate its test mode again in order to determine the cause of this problem. But this modification is not only kept secret by the chip manufacturer who has developed this electronic device, but also complex, costly and not reliable (because it requires a focused ion beam (or FIB) manipulation), in order to be performed only by this chip manufacturer.